A Study of Structural Methods and Concepts for Light Aircraft

J. position where he has to cover all facets of light air- plane design ... type of studies being performed by the Mission Analysis. Division. A few weeks ago, NASA ...
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A Study Of Structural Methods And Concepts For Light Aircraft (Text of presentation at the Experimental Aircraft Association Convention in Rockford, 111, on August 4, 1967, by L. Pazmany). INTRODUCTION

r i ' H E AMATEUR builder by necessity is in the unique J. position where he has to cover all facets of light airplane design, construction and flying. Very few professional aeronautical engineers share this situation, and if they do, it is almost certain that they are members of the EAA. We are fortunate also to count among our members, scientists and university professors who are working on advanced ideas in light aviation. A very successful two-place light airplane in production by Swedish and German firms was designed by one of our members in California. Another group of EAA members designed, obtained the FAA certification, and is now manufacturing "all fiberglas" light airplane floats at a lower cost than the conventional aluminum, and eliminating the old problem of corrosion. Amateurs designed and built "all fiberglas" gliders which are perhaps the first successful application of glass reinforced plastic in the construction of a flying vehicle. These are a few of the achievements by EAA members which come to my mind. We know of them through aeronautical publications or personal contact. Their experience could be shared by other amateurs as well as by the industry. In this presentation, I hope to make our members aware of the work which NASA is doing for the advancement of light aviation and also open the door for ideas which could be useful to this program. The general aviation market has been increasing steadily in the past few years. How this market expands in relation to the total transportation system of the next 15 to 20 years may well depend upon the extent to which light aircraft can compete with other forms of transportation such as the automobile. For such aircraft to perform a competitive transportation function their utility and safety must be improved and costs reduced. The NASA, through in-house and contractual studies, is attempting to identify critical areas in aerodynamics, propulsion systems, structural materials and concepts, etc., where additional research may increase the safety, utility, and economy of light aircraft. The Mission Analysis Division is currently engaged in analytical studies of light aircraft with the primary objective of defining the approaches having the most potential for future light aircraft of high utility. In performing these studies it is apparent that structural materials and concepts will have a


strong impact on utility, safety and economy, and decisions on structure cannot be made solely on a design basis. Pure structural design considerations do not completely define the merit of a particular structural material or concept. Factors such as maintenance, manufacturing technique and production quantities have significant effects on structural decisions. Various investigations, performed by industry and others, have indicated that the use of fiberglas reinforced plastic and aluminum honeycomb are two promising approaches to reducing airframe cost. However, such investigations have been mainly of an "ad hoc" nature and therefore do not provide the design and cost information required for the broad parametric type of studies being performed by the Mission Analysis Division. A few weeks ago, NASA awarded a contract to San Diego Aircraft Engineering, Inc., in San Diego, Calif., a company engaged in all aspects of aircraft engineering, where I am chief design engineer. This contract is to make a comparative evaluation, at two levels of technology, of a wide variety of structural materials and concepts applicable to light aircraft on the basis of general design and cost information; and to apply the more promising structural materials and concepts to the conceptual design of light aircraft, and to identify key problem areas where additional research may increase the potential of promising materials or concepts. This will be a nine month study of the utilization of a variety of materials and structural concepts for two levels of technology, "near term" and "potential" state of the art, for application to light aircraft. "Near term", i.e., five years from now, would require a relatively small research and development effort in contrast to "potential" which might be 15 years in the future. The term light aircraft is intended to include both fixed and rotary wing types. The study will be conducted in two phases. Phase I, conducted during the first four months, will require the investigation of the use of the widest possible variety of structural materials and concepts in the construction of light aircraft. The materials and concepts will be classified into the two levels of the state of the art, and evalu(Continued on next page)


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3. Minimum rate of climb at sea level not less than 1,000 fpm.


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ated with respect to design procedures, manufacturing, maintenance and cost. At the conclusion of Phase I, a report will be prepared presenting the materials and concepts studied with general design and cost data which will be applicable to future parametric studies. In Phase II, covering the remainder of the study, SAE will select the more promising materials and concepts for each time period and apply them to the conceptual design of a light airplane and a light helicopter. This Phase II will be a detailed analysis, including sensitivity studies, of the structural design, production, and maintenance of light aircraft. The initial activities of this study will be directed toward investigating a wide variety of potential structural materials and concepts for the two time periods. SAE will identify and justify the criteria used to determine the inclusion of a material or concept. Also, SAE will evaluate and compare the various materials and concepts on the basis of design, manufacturing, maintenance, and cost. This will be a general analysis of the effects of our study which will be used in determining the applicability of the different materials and concepts; such as, stress conceptions, fatigue and vibration, machinability, formability, fastening, compatibility of different materials, ease of repair, availability, aerodynamic cleanliness, costs, etc. On the basis of the evaluation and comparison as previously described, SAE will determine the most suitable regions of application for each material and concept for both primary and secondary structural components. The goal of Phase I will be to summarize the general design and cost information of potential structural materials and concepts which can be useful in guiding future light aircraft analysis and design. It will include all materials and concepts studied in Phase I and indicate their most suitable areas of application. At the beginning of Phase II, SAE will select the more promising materials and concepts for each time period and apply them to the detailed structural design of the conceptual light aircraft meeting the following requirements: AIRPLANE

The minimum performance with a pilot and three passengers (170 Ibs. each), 200 Ibs. of baggage, and enough fuel and oil for take-off, landing, and four hours normal cruise plus 30 minutes reserve will be: 1. Maximum speed at rated rpm at sea level not less than 152 knots. 2. Normal cruise at 5.000 ft. not less than 130 knots.

4. Service ceiling not less than 14,000 ft. 5. The stall speed at sea level not greater than 48 knots. 6. The take-off distance over 50 ft. will not be greater than 1,000 ft. HELICOPTER

The minimum performance with a pilot and three passengers (170 Ibs. each), 200 Ibs. of baggage, and enough fuel and oil for take-off, landing, and three hours normal cruise plus 15 minutes reserve will be: 1. Maximum speed at rated rpm at sea level not less than 105 knots. 2. Normal cruise at sea level not less than 87 knots. 3. Minimum rate of climb at sea level not less than 1,200 fpm. 4. Service ceiling not less than 14,000 ft. 5. Hover in ground effect not less than 8,000 ft. 6. Hover out of ground effect not less than 5,000 ft. PROPULSION

1. Engines will be of the reciprocating or turbine type developing not more than 250 bhp total and weighing not more than 380 Ibs. total. 2. Propellers may be either fixed or variable pitch. DIMENSIONS AND AREAS

1. The cabin must accommodate four persons and have an internal volume (excluding baggage space) greater than 112 ft.3 with a width not less than 3.5 ft. 2. The baggage volume must not be less than 16 ft.3 and accommodate four 9 in. by 21 in. by 31 in. suitcases. MISCELLANEOUS

1. The landing gear may be either fixed or retractable tricycle type. 2. The weight of fixed equipment will be 220 Ibs. The detail design of these conceptual aircraft, will be of sufficient depth to justify confidence in the results of this study and to indicate how the information generated in Phase I is used in the design procedure. The selections made will be subject to the approval by the NASA. The influence of aircraft configuration on structural design will be indicated by varying number and location of engines, body design, wing location for fixed wing designs, rotor arrangement for rotary wing designs, etc. Weight penalties due to type of joints, cut-outs, high lift devices and cabin pressurization will be indicated. The costs associated with designing, producing and maintaining the structure of a light aircraft, made from the selected materials and concepts, will be determined including inspection and insurance costs. These costs will be defined and substantiated in detail.


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The sensitivity of aircraft costs to production rates and structural lifetime will be shown through a unit cost breakdown. The development of the unit cost will be defined and substantiated. At the conclusion of Phase II, SAE will identify and justify critical research problems, the solution of which will lead to structural and economic improvements in light aircraft. STUDY CONSTRAINTS

The studies will be made for time periods of approximately 5 and 15 years from the present. The effects of assuming any advanced technology for either time period will be indicated. The design, materials and structural concepts will comply with the existing Federal Air Regulations applicable to the particular type of light aircraft. Any inadequacies or restrictions in these regulations which may degrade the potential of certain materials or concepts will be identified and possible modification to the regulations suggested and justified. The results of appropriate previous and current studies on materials and structural approaches should be considered in determining feasible structural concepts for light aircraft. All costs should be expressed in 1966 dollars. In the sensitivity studies, "near term" production

rates will range from 2,000 to 20,000 aircraft per year while the "potential" time period rates will be 5,000 to 100,000 aircraft per year. Lifetime of the aircraft structure will be 10,000 flight hours. The sensitivity of lifetime will be indicated by using 50 percent and 150 percent of the above value. At the completion of the study, SAE will summarize and report all of its findings, conclusions and recommendations, including representative designs of future aircraft, to NASA. This unclassified NASA report will subsequently be published by the U.S. Government printing office end will be available to U.S. industry, business and the public. This is the end of my formal presentation and I will be glad to answer your questions. In the near future, I will call on some of you personally at your company, university or basement, but please feel free to contact me any time at my work in San Diego Aircraft Engineering. EDITOR'S NOTE—EAA Designees in the U.S. are cooperating with NASA and San Diego Aircraft Engineering in gathering statistics and completing documents that will assist in furthering the development of the light plane. EAA feels honored to have been selected for this worthwhile project and owes a vote of thanks to its designees.

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